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The research also suggests that both men and women who hope to conceive should kick the habit.
"The results of the present study suggest a negative biological effect of smoking on spermatozoa DNA integrity," said the lead author of one study, Dr. Mohamed E. Hammadeh, head of the assisted reproductive laboratory in the department of obstetrics and gynecology at the University of the Saarland in Saar, Germany.
Research by Hammadeh and his colleagues showed that men who smoke heavily may experience fertility problems stemming from a drop in levels of a protein crucial to sperm development, as well as damage to sperm's DNA.
Both studies are published in the Sept. 8 online issue of Human Reproduction.
In the first study, Hammadeh's team compared sperm from 53 heavy smokers (more than 20 cigarettes a day) against that of 63 nonsmokers.
After three to four days of sexual abstinence, a single semen sample was taken from all study participants, to measure levels of two forms of a specific type of protein found in sperm, called protamines. According to the researchers, protamines are key players in sperm development, helping to spur on the process by which chromosomes are formed and packaged during cell division.
Hammadeh and colleagues found that in the smoking group, one form of protamine appeared at levels that were 14 percent below concentrations observed in the sperm of nonsmoking men. This was enough to constitute a form of "protamine deficiency" and, in turn, raise risks for infertility among the smokers.
What's more, smoking-linked "oxidative stress" appeared tied to an increase in damage to sperm DNA, the team reported.
According to Hammadeh, past attempts to clarify the relationship between cigarette smoking and male infertility have had trouble identifying a molecular mechanism underlying any such link. So he believes the new finding should help convince male smokers struggling with infertility to kick the habit.
"Because of the fact that cigarette smoke contains mutagens and carcinogens, there have been concerns that smoking may have adverse effects on male reproduction," Hammadeh noted. The new findings help bear that out, he said.
The second study was led by Dr. Claus Yding Andersen, a professor of human reproductive physiology at the University Hospital of Copenhagen in Denmark. It focused on the impact of maternal smoking during the first trimester of pregnancy upon the development of the male fetus.
In this case, the authors analyzed tissue from the testes of 24 embryos that had been aborted between 37 and 68 days following conception.
After classifying the prospective mothers according to smoking habits, the research team found that the number of so-called "germ cells" -- cells that develop into sperm in males and eggs in females -- were 55 percent lower in the testes of embryos obtained from women who smoked. This observation held regardless of the mother's alcohol and coffee consumption habits.
As well, embryonic levels of so-called "somatic cells" (those that go on to form other types of tissue) were 37 percent lower among those women who smoked.
In both the case of germ and somatic cells, drop-offs in levels appeared to be "dose-dependent," meaning that the more the prospective mother smoked, the lower the number of cells grown by the embryo.
Based on these findings early in fetal growth, Anderson and his colleagues conclude that the apparent impact of smoking on cellular production might continue in male offspring carried to term. And that could mean a higher risk of impaired fertility in sons.
According to the Danish team, their earlier research involving female embryos also revealed "germ cell" reductions of about 40 percent for embryos taken from women who smoked during pregnancy. This suggests that maternal smoking in pregnancy may harm the reproductive health of both male and female offspring.
"Our results provide health care professionals who talk to women who are considering conceiving, or have conceived just recently, with a 'here and now' argument to convince them to stop smoking," Anderson said. "Because the negative effect of smoking appears to take place right from conception and during the early days [of gestation], when the human embryo becomes differentiated into either a girl or a boy."
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